1. Field of the Invention
The present invention relates to an apparatus and/or method for providing voltage and current to a plurality of induction motors and, more particularly, operating one or more of the induction motors in a regenerative mode to supply current to at least one other of the induction motors under certain circumstances.
2. Related Art
A motor controller is utilized to provide voltage and current to an induction motor. In a normal mode of operation, the voltage and current sourced to the induction motor oscillate at a frequency which (together with a magnitude of the voltage) dictate a speed at which the motor rotates.
FIG. 1 illustrates a conventional configuration for providing voltage and current to an induction motor. The conventional configuration includes a conventional control unit 10, which provides command signals to a motor driver 12. The motor driver 12 often receives power from a DC source 14 and converts the DC power to the voltage and current for the induction motor 16. As the induction motor 16 requires AC voltage and current, and in this example the motor driver 12 receives DC power from source 14, the motor driver 12 shown in FIG. 1 is an inverter. In some cases, the motor driver 12 provides three-phase AC voltage and current to the induction motor 16 if required.
As induction motors produce very large in-rush currents (and low torque) at start up, the conventional control unit 10 often includes a current detection circuit which senses the current drawn by the induction motor 16 from the motor driver 12. The sensed current is utilized by the conventional control unit 10 to adjust the voltage and frequency provided by the motor driver 12 to the induction motor 16 such that excessive currents are not drawn by the induction motor 16.
U.S. Pat. No. 4,689,543 to David J. Hucker discloses a conventional control protocol for adjusting the voltage and frequency delivered from the motor driver 12 to the induction motor 16. The entire disclosure of U.S. Pat. No. 4,689,543 is incorporated herein by reference. At start up, the voltage and frequency commanded by the conventional command unit 10 are at a maximum and, as the current drawn by the induction motor 16 exceeds a permitted level, the voltage and frequency are reduced until a stable operating point is reached. Until the induction motor 16 accelerates, the current drawn decreases and the conventional control unit 10 increases the voltage and frequency until a desired operating point is obtained. The Hucker patent makes clear that the voltage and frequency are reduced in a way which prevents the induction motor 16 from entering a regenerative mode inasmuch as both the frequency and voltage are reduced linearly but not equally as a function of current (See FIGS. 3 and 4 of the Hucker patent).
Unfortunately, the conventional configuration for providing drive power shown in FIG. 1 is unsuitable for providing voltage and current to a plurality of induction motors over a single bus, particularly when some of the induction motors are rotating and others of the induction motors require high start-up currents. Indeed, if the control protocol utilized by the conventional control unit 10 and motor driver 12 of FIG. 1 were used to drive a plurality of induction motors over a single bus, all of the induction motors would have to be started at one time. If some of the induction motors were already rotating and others were not rotating, the rotating motors would have to be stopped and all motors started together. Furthermore, even if stopping all the induction motors were acceptable (which is often not the case) a current overload during start-up could cause a positive feedback condition resulting in severe overload of the motor driver 12. In particular, the conventional control unit 10 may attempt to reduce the voltage provided by the motor driver 12 to the induction motors in an attempt to reduce current draw during an overload. When the voltage is reduced in the rotating induction motors, their magnetization flux will likewise reduce, thereby increasing the slip in the induction motors. This has the unfortunate effect of increasing (not decreasing) the current drawn by the induction motors. The conventional control unit 10 might then attempt to further reduce the voltage in the induction motors, thereby creating a positive feedback condition and eventually critically overloading the motor driver 12.
One way of avoiding the positive feedback condition is to design the motor driver 12 such that an over-current condition will not be reached while the induction motors are starting up. Unfortunately, this would require sizing electronic components within the motor driver 12 to handle currents which are orders of magnitude greater than might otherwise be required during normal operation.
Accordingly, there is a need in the art for a new apparatus and method for providing voltage and current to a plurality of induction motors over a single bus which (i) does not require that a motor driver provide excessive amounts of start-up current to the induction motors; (ii) permits some induction motors to start while other induction motors are already rotating; and (iii) permits relatively smaller electronic components to be utilized in the motor driver for providing the voltage and current to the induction motors.
In order to overcome the disadvantages of the prior art, a motor controller in accordance with one aspect of the present invention includes: a driver stage operable to provide voltage and current to a plurality of induction motors over a single power bus; and a control unit operable to monitor the current drawn from the driver stage and cause one or more of the induction motors to operate in a regenerative mode to supply current to at least one other of the induction motors when the current drawn from the driver stage exceeds a threshold.
Preferably, the control unit is operable to monitor a frequency of the voltage from the driver stage and determine a ratio of the voltage to the frequency. The regenerative mode is preferably entered by commanding the driver stage to reduce the voltage and frequency to the one or more induction motors while substantially maintaining the ratio of the voltage to the frequency. Respective magnitudes by which the voltage and frequency to the one or more induction motors are reduced are preferably functions of the current drawn from the driver stage such that a desirable amount of regenerative current is produced. Regenerative current produced by the one or more induction motors operating in the regenerative mode may advantageously be utilized to source at least one induction motor requiring, for example, start-up current.
As the at least one induction motor requiring startup current reaches sufficient rotational speeds, an amount of current required by that induction motor will correspondingly reduce and eventually the regenerative mode will be terminated. This is accomplished by preferably increasing at least one of the voltage and frequency delivered to the induction motors such that the regenerative mode is terminated and the induction motors are accelerated.
According to another aspect of the present invention, a method of providing voltage and current to a plurality of induction motors over a single bus includes: sourcing voltage and current to the plurality of induction motors; determining whether the current drawn exceeds a threshold; and causing one or more of the induction motors to operate in a regenerative mode when the threshold is reached or exceeded such that regenerative current is supplied to at least one induction motor.
Other objects, features and advantages of the present invention will become apparent from the following description of the invention, which refers to the accompanying drawings.